The present invention relates to a thickness measuring apparatus for precisely measuring in nondestructive testing a thickness of a transparent body by an interferometric method, especially, the thickness of a very thin film.
A conventional thickness measuring method is known wherein a thickness of a very thin film is measured in accordance with a transmissivity of .beta.-rays or X-rays. However, with this method, there are safety problems. Another known conventional thickness measuring method is an inexpensive measuring method wherein two infrared rays having different wavelengths are transmitted through an object to be measured and a thickness of the object is measured in accordance with a ratio of the attenuation rates of these infrared rays. This method is described by M. Sawaguti, in "Thickness Measurement of Plastic Film by Infrared Sensor," Sensor Gijutsu, Vol. 2, No. 7, June 1982, pp. 45-51. However, this method cannot provide a high precision measurement. Still another thickness measuring method is described in Japanese Patent Disclosure (KOKAI) No. 58-139006 wherein a thickness of a thin film is measured by a phase difference between transmitted light and nontransmitted light.
Still another thickness measuring method is described by A. Sawada et al., in "Si Wafer Thickness Measurement Using Infrared Lichtshnitt Method," Shingaku Giho Vol. 81, No. 45 (EMC81-45), 1981, pp. 9-15. A conventional thickness measuring method using an interferometric gauge is described by J. P. Goedgebuer et al., in "Construction of an interferometric gauge system for thickness measurement in white light," Optics and Laser Technology, August 1978, pp. 193-196.
A method of measuring changes in the thickness of a glass plate by utilizing optical interference is described in Japanese Patent Disclosure (KOKAI) No. 54-155861. According to this method, (a1) a laser beam is irradiated on a surface of a transparent plate, (a2) interference fringes are formed by optical components reflected by the upper and lower surfaces of the plate, (a3) the plate is moved relative to a light source and a photodetector, and (a4) a moving direction of the interference fringes and an increase/decrease in the order of interference are detected, thereby continuously measuring a change in the thickness of the plate upon radiation of the beam.
According to this method, although the change in thickness of the plate can be measured, an absolute value of the thickness cannot, resulting in inconvenience.